Datasheet

LMD18245
www.ti.com SNVS110E –APRIL 1998–REVISED APRIL 2013
Figure 12. The Source Switches of the Power Bridge and the Current Sense Amplifier
Applications Information
POWER SUPPLY BYPASSING
Step changes in current drawn from the power supply occur repeatedly during normal operation and may cause
large voltage spikes across inductance in the power supply line. Care must be taken to limit voltage spikes at
V
CC
to less than the 60V Absolute Maximum Rating. At a change in the direction of the load current, the initial
load current tends to raise the voltage at the power supply rail (Figure 11) again. Current transients caused by
the reverse recovery of the clamp diodes tend to pull down the voltage at the power supply rail.
Bypassing the power supply line at V
CC
is required to protect the device and minimize the adverse effects of
normal operation on the power supply rail. Using both a 1 μF high frequency ceramic capacitor and a large-value
aluminum electrolytic capacitor is highly recommended. A value of 100 μF per ampere of load current usually
suffices for the aluminum electrolytic capacitor. Both capacitors should have short leads and be located within
one half inch of V
CC
.
OVERCURRENT PROTECTION
If the forward current in either source switch exceeds a 12A threshold, internal circuitry disables both source
switches, forcing a rapid decay of the fault current (Figure 13). Approximately 3 μs after the fault current reaches
zero, the device restarts. Automatic restart allows an immediate return to normal operation once the fault
condition has been removed. If the fault persists, the device will begin cycling into and out of thermal shutdown.
Switching large fault currents may cause potentially destructive voltage spikes across inductance in the power
supply line; therefore, the power supply line must be properly bypassed at V
CC
for the motor driver to survive an
extended overcurrent fault.
In the case of a locked rotor, the inductance of the winding tends to limit the rate of change of the fault current to
a value easily handled by the protection circuitry. In the case of a low inductance short from either output to
ground or between outputs, the fault current could surge past the 12A shutdown threshold, forcing the device to
dissipate a substantial amount of power for the brief period required to disable the source switches. Because the
fault power must be dissipated by only one source switch, a short from output to ground represents the worst
case fault. Any overcurrent fault is potentially destructive, especially while operating with high supply voltages
(≥30V), so precautions are in order. Sinking V
CC
for heat with 1 square inch of 1 ounce copper on the printed
circuit board is highly recommended. The sink switches are not internally protected against shorts to V
CC
.
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